1. Field of the Invention (Technical Field):
The present invention relates to methods and apparatuses for prevention of vibration in drillstrings used for drilling/earth-boring applications.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Chatter and other forms of vibration cause failure of the cutter portion of drillstrings and concomitant costs in parts and repair time (during which drilling is halted). In particular, failures of Polycrystalline Diamond Compact (“PDC”) cutters and damage to the bottom hole assembly are of concern, which often occur in hard rock such as Sierra White Granite (“SWG”). Accordingly, a great deal of effort has been expended in minimizing vibrations of various types in drillstrings. Of particular concern are self-excited vibrations (chatter) in the longitudinal direction, and some work has been done on detection and suppression of chatter in drillstrings, see, e.g., Y. Tarng, et al., “Adaptive Pattern Recognition of Drilling Chatter”, Journal of Materials Processing Technology 48:247–253 (1995); Y. Tarng, et al., “Detection and Suppression of Drilling Chatter”, Journal of Dynamic Systems, Measurement, and Control 116:729–734 (1994); and Y. Tarng, et al., “On-Line Drilling Chatter Recognition and Avoidance Using an ART2—A Neural Network”, Int. J. Mach. Tools Manufact. 34:949–957 (1994), although without great success.
Even though there are many types of vibrations encountered during drilling, such as bit whirl, chatter has only recently received attention as one of the main causes of failure in PDC bits, particularly in hard rock formations such as SWG. See, e.g., M. Elsayed, et al., “Effect of Downhole Assembly and Polycrystalline Diamond Compact (PDC) Bit Geometry on Stability of Drillstrings”, ASME Journal of Energy Resources Technology 119:159–163 (1997). Sandia National Laboratories has led an effort to investigate the cutting forces in PDC bits, D. Glowka, “Use of Single-Cutter Data in Analysis of PDC Designs: Part 2—Development and Use of PDCWEAR Computer Code”, Journal of Petroleum Technology pp: 850–859 (August, 1989) and recently by measuring chatter during drilling using their Hard Rock Drilling Facility (“HRDF”) showed the detrimental effect of chatter on the life of PDC bits. M. Elsayed, “Correlation Between Bit Chatter, Weight on the Bit and the Rate of Penetration in Drillstrings Equipped with PDC Bits”, ASME ETCE Conference Proceedings, Houston, Tex., pp. 1–8 (1999); and M. Elsayed, et al., “Measurement and Analysis of Chatter in a Compliant Model of a Drillstring Equipped with a PDC Bit”, ASME ETCE Conference Proceedings, New Orleans, La., pp. 1–8 (2000). Even though it was shown that the proper combination of weight on bit (“WOB”), rotating speed and bit design reduces the axial chatter, added means of controlling these vibrations under field conditions are needed. This is particularly important since the dynamics of drillstrings are constantly varying with the drilling depth. Moreover, rock properties vary and unexpected variations in rock hardness is common. Sudden changes in load as the bit transitions a soft-to-hard rock boundary is often sufficient to induce drillstring instability.
Shock absorbers, otherwise referred to as shock subs, have been used in drillstrings to suppress vibrations. Also, fluids (mainly “passive” fluids, i.e., those whose properties are not controlled once installed in the shock sub) have been used to provide friction (damping) in these devices. Technology in this area includes: U.S. Pat. No. 6,394,198, to Hall et al., entitled “Frictional Vibration Damper for Downhole Tools”; and U.S. Pat. No. 6,364,775, to Rohs et al., entitled “Torsion Vibration Damper and Process for its Manufacture”.
MR fluids have been used in conjunction with a drillstring in U.S. Pat. No. 6,257,356, to Wassell, entitled “Magnetorheological Fluid Apparatus, Especially Adapted for Use in a Steerable Drill String, and a Method of Using Same” (filed Apr. 27, 2001), U.S. Patent Publication No. 2002/0011358, to Wassell, entitled “Steerable Drill String” (Jan. 31, 2002), and PCT Application Ser. No. US01/25586, to Wassell, entitled “Steerable Drill String” (Apr. 12, 2001). These disclose use of MR fluid in a device to steer the drillstring to drill in a given direction, with no mention or suggestion of vibration suppression capability.
The present invention introduces damping at the cutting end of the drillstring to reduce chatter and other vibrations via controllable Magneto-Rheological (“MR”) fluid dampers coupled with the altering of the damping constant through variation in a control voltage. Reduction in bit vibration also results in an increase in the rate-of-penetration (“ROP”), with resulting increase in drilling efficiency and reduction in drilling cost.